When tilted back (images at centre and right), the head valve i is semicircular  with the articulamentum extending all round the curved anterior-lateral edge, where 20 slits cut it into rectangular teeth which insert into the surrounding girdle. On the ventral surface a pale slit-ray with round pores leads to each of the 20 slits. In dorsal view with the curved edge on the substrate (image on left), the posterior margin of valve i is widely V-shaped. Orkney, Scotland, March 1976.